Why measure personal UV radiation? Solar ultraviolet (UV) light reaching the earth's surface is radiation in the wavelength range of 280-400 nm. It has critical impact on humans. The skin synthesizes Vitamin D using UV exposure, which makes it necessary for health. But overexposure to UV can cause adverse effects such as immune reactions in lupus, sunburns, or phototoxicity in the short term, and skin cancer in the longer term. Sensitivity to ultraviolet radiation varies from person to person (e.g., darker skin types are at lower risk for sunburn), from disease to disease, from pharmaceutical treatment to pharmaceutical treatment, etc. Reaction to UV depends not only on the instantaneous strength of the UV radiation, but also the time over which a person is exposed to it. The accumulated UV radiation over time is called the UV dose.
Sun-related activity varies from individual to individual. Outdoor runners are more exposed to UV than indoor treadmill runners. Certain professions, such as construction, involve large exposure to UV on a daily basis, while traditional office jobs involve lower UV exposure. Under such circumstances, the primary way to be able to control any symptom of overexposure (such as sunburn, immune reaction, and phototoxicity), is to have an accurate knowledge of personal UV dose. US20170115162A1 describes exemplary systems and methods that include a wearable device that is adapted to measure and aggregate the UV exposure information, while a mobile device is adapted to display metrics and alerts to the user based on this information.
How to measure UV radiation? In 1987, the human sensitivity to ultraviolet radiation was defined by Diffey and later adopted by the World Metereological Organization and the World Health Organization. This sensitivity is called the “Erythema action spectrum” and gives exponentially more importance to high-energy photons. This standard metric is called the ultraviolet index (UV Index, or “UVI”) when ultraviolet exposure is measured on a horizontal plan.
Typical UV measuring systems include a UV measuring diode (or a set of them), which converts the incident ultraviolet radiation signal to electric current, coupled with additional circuitry. This includes an analog-to-digital converter (ADC), op-amp and microcontroller (Amini N., Matthews E. J., Vandatpour A., Dabiri F., Noshadi H., Sarrafzadeh M., “A Wireless Embedded Device for Personalized Ultraviolet Monitoring,” International Conference on Biomedical Electronics and Devices, pp. 200-205 (2009)). Some examples of these systems are the Solarmeter® 6.5 UVI and the Genicom UV Index Meter.
Why is accuracy important? Many human bodily functions are dependent on UV, while several conditions are activated by it. For some of these conditions, such as phototoxicity and photocarcinogenicity, it is important to know the activation threshold—and it varies from person to person. For others, such as sunburn, safety thresholds are known according to skin type, but it is vital to know current exposure relative to such thresholds. Overestimation can lead us to determine higher thresholds than reality, which leads to dangerous overexposure. Underestimation can lead to longer periods spent out in the sun, which can easily cause sunburn. This makes accuracy of extreme importance in the measurement of UV exposure.
An approximate forecast for the maximum daily UV Index is usually provided by local weather services, but is largely inadequate for measuring personal UV exposure. It assumes the orientation of the person to be flat on the ground, which is the case only very infrequently. Further, it does not account for the presence of reflective environments like water and sand, which can greatly increase UV exposure, or the variation of the UV index over the course of the day with time and weather. Finally, it does not take into account secondary radiation in the shade or through windows.
What is calibration? The disclosure herein on calibration applies generally to a wide variety of radiometer systems, whether they are referenced herein or not. Some aspects of the calibration concepts herein, however, may be related to UV sensing devices, or at least some functionality of UV sensing devices, which may be expressly described herein. The disclosure herein related to calibration is thus not limited to particular UV sensing devices expressly provided herein, however some calibration methods or concepts herein may be enabled by one or more of the UV sensing devices herein.
The input to exemplary radiometer systems is the solar spectrum, which can be represented as the spectral irradiance s(λ). Spectral irradiance is the radiant flux received per unit area per unit wavelength, and is represented in the units of Wm−2nm−1 (http://www.pveducation.org/pvcdrom/properties-of-sunlight/spectral-irradiance). The output of the radiometer (pmeasured) is the spectral response of the radiometer r(λ) convolved with this input spectral distribution (Eq. 1).pmeasured=∫λ=−∝∝s(λ)r(λ)  Eq. (1)
The actual value of the property is the ideal response is {circumflex over (r)}(λ) convolved with the same spectral distribution s(λ) (Eq. 2). Some examples of ideal response are shown in FIG. 1A below.pideal=∫λ=−∝∝s(λ){circumflex over (r)}(λ)  Eq. (2)
The calibration process adjusts for two main problems: 1) Manufacturing variations, and 2) the difference between the ideal response required by the radiometer's specifications and the actual spectral response of the radiometer. Calibration usually utilizes a least squares minimization on a set of known values (pknown) versus measured (pmeasured) (values to adjust pmeasured (Eq. 3):r(λ)=argminr(λ)∥Pmeasured−Pknown∥  Eq. (3)
How does calibration impact accuracy? Proper calibration of a radiometer requires its traceability to a governmental agency of standards, such as the National Institute of Standards and Technology in the United States or the Physikalisch-Technische Bundesanstalt in Germany. To claim such traceability, radiometers need to be calibrated against a traceable calibrated source, a traceable calibrated spectroradiometer or radiometer, or both. Calibration ensures that variability in the manufacturing does not affect the measurement.
Typically, calibration is performed after manufacturing of the instrument and thereafter every year, usually using a stable electromagnetic source, i.e., with a single pknown and pmeasured. An ideal spectral response cannot be achieved in practice, and if such ideal behavior is required for the radiometer's measurement (e.g., the radiometer is required to measure the UV index)—which is the case for most radiometers—this calibration method does not guarantee the accuracy of radiometers in different environments, where the spectra to be measured are different from the calibration spectrum. The error incurred as a result is called the spectral mismatch error (see Xu, G. & Huang, X. Characterization and calibration of broadband ultraviolet radiometers. Metrologia 37, 235-242 (2003)). This may be acceptable for applications where the focus is on measuring the radiant flux of a source for which the spectral distribution is known, e.g., a UVA radiometer used to measure photoresist exposure in ultraviolet lithography is calibrated using the same source that is used for lithography. However, when sensing personal UV exposure, the radiometer/dosimeter is meant to measure UV irradiance from solar spectra, which varies greatly according to location and time of the day.
The US Environmental Protection Agency (EPA) UVNET database illustrates how spectral distributions vary from location to location. This is a publicly available database containing Brewster spectrometer scans from various locations in the USA, taken at different times of the year, on multiple years. The included locations are as far north as Acadia National Park, Me. to as far south as Big Bend, Tex., and hence cover a broad range of latitudes. Locations at altitude such as Albuquerque, N. Mex. (5000 feet above sea-level) are also included. The Brewster spectrometer scans wavelengths from 286 nm to 363 nm. FIG. 1A shows three scans taken at approximately local noon on Sep. 3, 2000 at three different locations. This spectral data was obtained from Environmental Protection Agency, U. Ultraviolet and Ozone Monitoring Program: UV-Net. (2008). At http://www.epa.gov/uvnet/. The top scan is from Big Bend, Ill., the middle scan is from Albuquerque, N.Mex., and the bottom scan is from Acadia NP, Me. The difference in the spectral distributions is evident. FIG. 1B shows spectral scans on different days of the year from Big Bend, Tex. at local noon, and illustrates that similar differences are observed at the same location on different days of the year at the same time. UV radiometer/dosimeter systems that are measuring personal UV exposure need to measure UV irradiance from all these different situations accurately. To accomplish this, calibration using a single source that is non-representative of the sun is inadequate.
FIG. 2 further shows the solar spectrum in comparison to that of a deuterium arc lamp, which is typically used in calibrating UV radiometers (the fairly straight line is that of the deuterium arc lamp, while the other line is the solar spectrum). It is evident from FIG. 2 that this calibration source greatly differs from the solar spectrum in the UV region, which means a UV radiometer calibrated using the arc lamp source is very likely to be inaccurate when measuring solar UV.
Existing Approaches. There have been a few proposed systems for calibrating radiometers, such as those described in U.S. Pat. Nos. 6,729,756, 4,726,688, and 6,439,763, which are incorporated by reference herein. U.S. Pat. No. 6,729,756 describes a method for calibrating a microwave radiometer for a satellite. This utilizes a hot and cold calibration source. Such radiometers are much higher power and measure much larger wavelengths compared to UV radiometers and dosimeters being used to sense personal UV exposure, and hence the calibration apparatus is much larger and different in nature. U.S. Pat. No. 4,726,688 describes measuring infra-red radiation from sunlight. The calibration source consists of a heater attached to a sensing plate, and is very different from some calibration sources described herein, which include a plurality of light sources, such as a plurality of light-emitting diodes (“LEDs”). The third system in U.S. Pat. No. 6,439,763 describes radiometers measuring thermal radiation, which is not directly relevant to the proposed UV measuring systems herein.
The idea of custom calibration has been proposed as early as 1991, such as in U.S. Pat. No. 5,204,532. This method describes a custom calibration for infra-red based blood glucose sensors, which can be tuned to each individual user. This is a necessity for the field because different individuals have different base energy absorption rates, depending on water, fat and protein content in the body. For UV sensing, a general calibration may be sufficient, but a custom calibration based on location information can improve accuracy. One exemplary method of improving accuracy is by clustering a plurality of calibration functions based on binning of location parameters (e.g., solar zenith angle), which is described below.
U.S. Pat. No. 7,230,222 described the idea of using multiple light-emitting diodes (LEDs) to form a calibration source to mimic a single spectrum constituting white light. This disclosure, however, fails to describe or teach how to make a calibration source programmable so that it can, for example, replicate or mimic a plurality of spectra, which is described herein below. U.S. Pat. No. 6,967,447 discusses using multiple light-emitting sources with different current drivers so as to be programmable. However, it fails to teach how to derive the current for each light source to form a spectrum, which is something that the disclosure below provides in detail. The disclosures of U.S. Pat. Nos. 7,230,222 and 6,967,447 related to electronics, circuitry, and components, however, are fully incorporated by reference herein and by incorporated into embodiments herein.
Base on the deficiencies of these existing systems, there is a need for improved methods and systems for calibrating UV radiometers and UV dosimeters to enhance the accuracy of the measurements.